SIP User Agent

A SIP endpoint that initiates (UAC — User Agent Client) or receives (UAS — User Agent Server) SIP requests, typically a VoIP phone, softphone, or WebRTC browser application. The same device acts as both UAC and UAS in different transactions — a phone is a UAC when calling and a UAS when receiving. User agents maintain dialog state and are responsible for authentication, codec negotiation, and media session management.